1. Field of the Invention
The present invention relates to an optical recording medium and an ultraviolet-curable composition used for forming a protective film of that medium, and more particularly, to an optical recording medium, in which the reflecting film is Ag or a metal and/or alloy having Ag for its main component, and an ultraviolet-curable composition used as its protective film.
2. Description of Related Art
Write-once, rewritable and various other types of optical recording media, and particularly optical recording discs, have attracted attention in recent years as large-capacity data recording media. In particular, optical recording disc, which has a recording layer having a dye for its main component, can be produced at low cost since the recording layer is able to be formed by coating. These optical recording discs have been observed to demonstrate increasing popularity throughout the world.
Although optical recording discs employing a so-called air sandwich structure, in which an air layer is provided on the recording layer, have been used as optical recording discs in the past, more recently, optical recording discs have been developed that are capable of reading in accommodation of compact disc (CD) standards by providing a reflective layer adhered to the surface of the recording layer. Namely, these optical recording discs are comprised of sequentially laminating a recording layer having a dye for its main component, reflective layer and protective layer in that order on a polycarbonate or other transparent substrate, providing the reflective layer adhered to the recording layer, and enabling a constitution having a total disc thickness of 1.2 mm in accommodation of CD standards. Moreover, similarly laminated optical recording discs that comply with DVD standards compatible with high density have also been proposed.
In optical recording discs of this type of layer composition, a chemically stable Au (gold) thin film has been used in the past for the reflective film in order to comply with CD reflectance standards. However, accompanying the proliferation of optical recording discs, there have been greater demands placed on reducing cost. As a result, reflective films composed of Ag or a metal and/or alloy having Ag for its main component are coming into the mainstream in order to produce less expensive discs. On the other hand, Ag thin films have the disadvantage of having poor corrosion resistance. Namely, although Ag is not oxidized even when heated to high temperatures and is a chemically stable metal in a dry atmosphere, in the presence of air containing a normal amount of moisture or in the presence of sulfide gas, Ag encounters problems in which it deteriorates, becomes discolored or loses its luster. Thus, it becomes necessary to improve protective performance of the protective film that protects it to an even greater extent than in the case of using an Au thin film for the reflective film.
In the past, techniques have been proposed for alloying Ag with various types of metals in order to improve the corrosion resistance of Ag thin films when used as reflective films. For example, the containing of Mg (magnesium) at 1-10 wt % is proposed in Japanese Unexamined Patent Application, First Publication No. 7-3363. However, although these techniques are observed to be slightly effective, in the case of an optical recording medium in which a protective film is formed with an ultraviolet-curable composition of the prior art, durability under conditions of high temperature and high humidity was not considered to be adequate.
On the other hand, with respect to ultraviolet-curable compositions used as protective films, the containing of a phosphate ester monomer is known to be effective in the case of a using an Al (aluminum) thin film or other metal thin film of the prior art for the reflective film (e.g., Japanese Unexamined Patent Application, First Publication No. 62-97150 and Japanese Unexamined Patent Application, First Publication No. 3-131605). However, in the case of using a cured film of an ultraviolet-curable composition based on these findings for the protective film, adequate durability of the optical recording medium under conditions of high temperature and high humidity is unable to be obtained in the same manner as previously described.